We don't know if it really was a regular tau fruit or some freak fruit that just looked like a normal one. Main would need to test it out a few times at least before jumping into conclusions. It seems pretty strange nobody would know of it if it was 100% systematic.

I'd guess Main is still sickly because she was so weak for such a long time. Perhaps she will need more time to build stamina up to normal human levels. If it's not that, then it means there's something more wrong about her status. I bet the higher nobles would know more, but there's no way they would tell.